Presintering effects on ground-based and microgravity liquid phase sintering
- PDF / 2,186,483 Bytes
- 11 Pages / 612 x 792 pts (letter) Page_size
- 2 Downloads / 268 Views
I. INTRODUCTION
LIQUID PHASE sintering is a common process for the fabrication of high-performance, net-shaped structures. During liquid phase sintering, a liquid phase coexists with a particulate solid at the sintering temperature. The fast atomic diffusion in the liquid phase, coupled with the capillary force, promotes rapid compact densification.[1] But the presence of excessive liquid often induces undesired shape distortion and microstructural inhomogeneity. From a technical point of view, the major advantage of liquid phase sintering is the fast densification at low sintering temperatures. However, liquid phase sintering is primarily limited to high solid volume fractions, due to the lack of contacts between the solid particles at a high liquid content.[2,3] Distortion at the macroscopic level is related to composition. Johnson et al.[4] reported that the W-Ni-Fe alloy distorts when its solid-volume content is less than 60 pct, while the W-Cu alloy can be sintered without distortion with a 20 pct solid volume. These distortion observations have been linked to microstructural features such as the solid volume fraction, contiguity, and dihedral angle.[5,6] Compositions with a high solid content, high contiguity, and large-dihedral-angle systems have more structural rigidity and exhibit less distortion. According to previous research,[7] a minimum contiguity of 0.38 is required for shape preservation. Based on this contiguity criterion, there is a critical solid volume fraction necessary to maintain rigidity for any given liquid phase sintering system. For tungsten heavy alloys, the large density difference between the tungsten solid phase and the liquid phase makes it a model system for distortion and microstructural evolution PEIZHEN LU, Director of Materials Processing, and RANDALL M. GERMAN, Brush Chair Professor in Materials, are with the Center for Innovative Sintered Products, P/M Lab, The Pennsylvania State University, University Park, PA 16802-6809. R.G. IACOCCA, formerly Associate Professor with the Center for Innovative Sintered Products, P/M Lab, The Pennsylvania State University, is with Eli Lilly & Co., Indianapolis, IN 46285. Manuscript submitted September 14, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
studies. Active research on distortion and densification in liquid phase sintering has been conducted on the W-Cu,[8] W-Ni,[9,10] W-Ni-Fe,[5,7] and W-Ni-Cu[11] systems. For these alloys, the sintering behavior is sensitive to the processing conditions. As an example, strength and ductility exhibit a high sensitivity to residual porosity, especially at levels greater than 0.5 pct.[12] To effectively eliminate pores, the sintering atmosphere and presintering conditions need to be properly chosen. Studies[13,14,15] have found that a long sintering time in dry hydrogen gives a high residual porosity and poor sintered properties. German and Churn[12] studied sintering-atmosphere effects in stabilizing detrimental residual pores, showing that the pore degassing due to gas pressurization during den
Data Loading...
